Heiße Spuren (1974)

Ten-year-old Lutz Paschke, son of detective captain Paschke, is an enthusiastic but often overzealous detective. He roams his town with his basset hound Pinkus and thinks he can spot crimes in mostly harmless incidents. Every now and then, his father is called by him to a supposed crime, so Lutz thinks he has seen a murder, which turns out to be a clumsy attempt to kill a carp. Father Paschke is annoyed, especially as Lutz's actions repeatedly put him in embarrassing situations. So when Lutz receives an anonymous letter with cut-out letters announcing a crime at the old mill on Friday evening, he doesn't tell his father.